Just my 2 cents.
was thinking sasuke is possessed by black zetsu. same way white zetsu was in him, orochimaru etc. and hence makes him evil. he is about to respawn the evil mother.
your opinins
why i ask is because it was pretty disappointing her being sealed in that way. nagato could hold his own and if it was him in that scene he would have easily shinra tensei kakashi, naruto, sasuke and even sakura. even to kill naruto/ sasuke y separate them i believe nagato would done that easily...
i have feeling she remained behind to check out the rikudo tools... she is a tool user after all.
and those will come in handy to defeat madara seeing that taijutsu and senjutsu are not working...